The role of auxin in nitrogen-modulated shoot branching

ABSTRACT

Shoot branching is determined by axillary bud formation and outgrowth and remains one of the most variable determinants of yield in many crops. Plant nitrogen (N) acquired mainly in the forms of nitrate and ammonium from soil, dominates plant development, and high-yield crop production relies heavily on N fertilization. In this review, the regulation of axillary bud outgrowth by N availability and forms is summarized in plant species. The mechanisms of auxin function in this process have been well characterized and reviewed, while recent literature has highlighted that auxin export from a bud plays a critical role in N-modulating this process.

Nitrogen and shoot branching

Plants continuously adjust development to suit the environmental conditions through the life cycle. Shoot branching is an excellent example of such developmental plasticity and determines grain yield especially in cereal crops.¹ The number of branches (or tillers in rice and wheat) is mostly determined by whether axillary buds are formed in the axils of leaves, and whether they are inhibited or released to elongate.²

Nitrogen (N) is the most important mineral nutrient in plants and regulates their development including shoot branching. Several studies have reported the impact of N availability on shoot branching in plant species.^3–7 And the tiller number is positively correlated with the N input under a certain N level in rice.^8,9 Besides N availability, tiller number in rice was also affected by N forms,^10–12 which wasn’t found in other species such as poplar and Arabidopsis.^3,4 The shoot branching is affected by N availability and forms mainly through axillary bud elongation rather than its initiation.^4,10,12

The regulation mechanism of N on plant architecture has been partially elucidated during the past decades. Increasing or decreasing the expression of N-related transporters and assimilation genes can consequently affect shoot branching in plant species.^13–18 Moreover, a recent study found N stimulation of tillering in rice is regulated by NGR5 (N-mediated tiller growth response 5),¹⁹ in which N-induced NGR5 inhibits expression of the shoot branching-inhibitory genes OsD14 and OsSPL14. However, direct additions of nutrient to inactive lateral buds do not stimulate their outgrowth,^3,20 while the addition of nutrient through the root systems of deprived plants often has remarkable promotive effects on bud outgrowth,^21–25 suggesting N may indirectly act to regulate branching.^5,26–28

Auxin regulation of shoot branching

The mechanism controlling shoot branching has been extensively studied that auxin is the main player involved in regulation of axillary bud outgrowth.²⁹ Auxin is mainly produced in the young leaves at the shoot apex³⁰ and transported downward in the polar auxin transport stream (PATS), but does not enter the bud and therefore acts indirectly on bud outgrowth.^31,32 The PATS mainly depends on the function of auxin efflux facilitators of the PIN-FORMED (PIN) family.^33,34 The unifying models explain PATS control of bud outgrowth triggered by removal of the shoot apex, which are mainly based on research conducted in Arabidopsis and pea plants. One proposed mechanism by which auxin in the PATS can inhibit buds indirectly is by regulating the synthesis of second messengers, such as strigolactones, which can move into buds to modulate their activity.³⁵ Another explanation is that PATS, which provides a high conductance auxin transport route down stems dominated by the auxin export protein PIN-FORMED1 (AtPIN1), leads to sustained outgrowth of the axillary buds.^36–39 Recent research has shown that AtPIN3, AtPIN4 and AtPIN7 also contribute to the communication between the PATS and surrounding tissues such as axillary buds.³⁸ Similarly, in cucumber, CsPIN3 is supposed to function in auxin export from axillary buds because suppression of its expression leads to auxin accumulation in buds.⁴⁰ The genes directly or indirectly involved in the N and auxin regulation of shoot branching are summarized in Table 1.

Table 1.

Genes were directly or indirectly involved in the N and auxin regulation of shoot branching in plants

Gene	Gene locus	Protein type	Responding to N/auxin	Effect on shoot branching	Reference
OsTCP19	LOC_Os06g12230	TCP transcription factor	repressed by N availability	reduces tiller number	41
OsNGR5	LOC_Os05g32270	APETALA2-domain transcription factor	induced by N availability	increases tiller number	19
OsLHT1	Os08g0127100	Lysine-Histidine-type Transporter 1	unknow	increases tiller number	42
OsAAP5	LOC_Os01g65660	Amino acid permease	unknow	reduces tiller number	43
OsASN1	Os03g0291500	Asparagine synthetase	responds to ammonium	increases tiller number	44
OsAAP3	LOC_Os06g36180	Amino acid permease	unknow	reduces tiller number	16
OsMADS57	LOC_Os02g49840	MADS transcription factor	induced by nitrate	induces outgrowth of axillary bud	45,46
OsNRT1.1B	LOC_Os10g40660	NItrate transporter	induced by nitrate	increases tiller number	15
OsNPF7.1	LOC_Os07g41250	Member of the NPF family	regulated by external N	promotes axillary bud growth	47
OsNPF7.2	LOC_Os02g47090	Low-affinity nitrate transporter	induced by high nitrate	promotes axillary bud growth	48
OsNPF7.3	LOC_Os04g50950	Peptide transporter	induced by organic N	increases tiller number	49
OsNPF7.7	LOC_Os10g42870	Putative nitrate transporter	suppressed by high N	promotes axillary bud growth	50
OsGS1;2	LOC_Os03g12290	Glutamine synthetase	induced by ammonium	promotes axillary bud outgrowth	51,52
TaGS2-2Ab	GQ169685	Glutamine synthetase	induced by high nitrate	increases tiller number	53,54
OsNR2	LOC_Os02g53130	Nitrate reductase	induced by nitrate	increases effective tiller number	52,55
OsCEP6.1	LOC_Os08g37070	Mature post-translationally modified peptide of 15 amino acids	induced by low N	reduces tiller number	56
OsPIN1	LOC_Os02g50960	Auxin efflux carrier	induced by nitrate	reduces tiller number	57
OsPIN2	LOC_Os06g44970	Auxin efflux carrier	induced by nitrate	increases tiller number	58
OsPIN3t	LOC_Os01g45550	Auxin efflux carrier	induced by nitrate	reduces effective tiller number	59
OsPIN5b	LOC_Os08g41720	Auxin efflux carrier	induced by nitrate	reduces tiller number	60
OsPIN9	LOC_Os01g58860	Auxin efflux carrier	induced by ammonium	increase tiller number	12
CsPIN3	Csa5G576590	Auxin efflux carrier	unknow	promotes axillary bud growth	40
AtPIN1	At1g73590	Auxin efflux carrie	unknow	promotes axillary bud growth	61
AtAXR1	At1g05180	RUB1 activating enzyme	responds to auxin	supresses branching	62

Auxin function in N-regulated shoot branching in plants

Increasing evidence has revealed that N fluctuations have a significant impact on auxin distribution in plants. A decrease in N supply commonly increases indole-3-acetic acid (IAA) accumulation in the root of plants including Arabidopsis, soybean, durum wheat, and maize.^62–64 However, decreasing the N supply of rice plants from 2.5 mM to 0.01 mM results in a 30% lower IAA content in the junction and root.⁶⁵ Discrepancy of auxin distribution responding to N supplies might result from varying experimental condition such as N condition and plant species. We observed that the highest expression of DR5:GUS expression was detected in roots of several rice species when N concentration increasing from 0 to 1–2 mM, and afterward decreased when N continuing to increase up to 5 mM (data unpublished). The mechanisms in which auxin distribution in plant tissues is affected by N availability have been reported to be involved in transcription factors.⁶⁶ For example, a MADS-box transcription factor, OsMADS57, participates in auxin distribution in roots of rice plants and thus modulates seminal root elongation.⁴⁵

The establishment of auxin distribution within plant tissues constitutes its function in plant morphogenesis. Studies have supported that N-regulated PATS is involved in plant root development.^63,66,67 The ability of plants to activate branching requires high conductance auxin transport and PIN polarization between the bud and the main stem in Arabidopsis.^37–39 Although the rice tillering pattern is obviously different from Arabidopsis, results in rice support the general conservation of regulatory mechanism, in which ammonium induces rice tiller bud outgrowth in comparison with nitrate, which results from enhanced³H-IAA export from the tiller stem.¹² Additionally, OsPIN9, a functional auxin-efflux transporter which is mainly expressed in the stele of axillary bud and induced by ammonium rather than nitrate, plays an important role to establish high conductance auxin transport from axillary bud (Figure 1). More interestingly, overexpression lines of OsPIN9 had a greater advantage of grain yield in the paddy field at a low-N rate than at a high-N rate, resulting from more increased tiller numbers. Results in rice suggest that OsPIN9 is a novel factor which contributes to branching specifically in grasses and can be used in the molecular breeding of rice varieties to reduce N fertilizer input in the future.

Figure 1.

A working model of OsPIN9 in modulating tiller bud outgrowth in rice. The main shoot tip produces massive auxin, which moves downward to the junction and roots. Overexpression/mutation of OsPIN9 can induce/inhibit auxin efflux from tiller buds, besides auxin polar transport in main stem, which leads to active/inactive tiller bud outgrowth. Interestingly, ammonium (NH₄⁺) induces OsPIN9 expression and consequently affects tiller bud elongation in comparison with nitrate (NO₃⁻). Black and blue solid lines mean higher OsPIN9 expression and auxin transport. Black and blue dotted lines present lower OsPIN9 expression and auxin transport

Funding Statement

This work has been funded by the National Key R&D Program of China (2018YFD0200503), the National Nature Science Foundation of China (31972501 and 31672225), Innovative Research Team Development Plan of the Ministry of Education (IRT_17R56; KYT201802), 111 Project (number 12009)..

Disclosure statement

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