The YDA-MKK4/MKK5-MPK3/MPK6 Cascade Functions Downstream of the RGF1-RGI Ligand-Receptor Pair in Regulating Mitotic Activity in Root Apical Meristem

The mitotic activity of root apical meristem(RAM)is critical to primary root growth and development.Previous studies have identified the roles of ROOT GROWTH FACTOR 1(RGF1),a peptide ligand,and its receptors,RGF1 INSENSITIVEs(RGIs),a clade of five leucine-rich-repeat receptor-like kinases,in promoting cell division in the RAM,which determines the primary root length.However,the downstream signaling components remain elusive.In this study,we identify a complete mitogen-activated protein kinase(MAPK or MPK)cascade,composed of YDA,MKK4/MKK5,and MPK3/MPK6,that functions downstream of the RGF1-RGI ligand-receptor pair.Similar to the rgi1/2/3/4/5 quintuple mutant,loss-of-function mutants of MPK3 and MPK6,MKK4 and MKK5,or YDA show a short-root phenotype,which is associated with reduced mitotic activity and lower expression of PLETHORA 1(PLT1)/PLT2 in the RAM.Furthermore,MPK3/MPK6 activation in response to exogenous RGF1 treatment is impaired in the rgi1/2/3/4/5 quintuple,yda single,and mkk4 m kk5 double mutants.Epistatic analyses demonstrated that the expression of constitutively active MKK4,MKK5,or YDA driven by the RGI2 promoter can rescue the short-root phenotype of the rgi1/2/3/4/5 mutant.Taken together,these results suggest that the YDA-MKK4/MKK5-MPK3/MPK6 cascade functions downstream of the RGF1-RGI ligand-receptor pair and upstream of PLT1/PLT2 to modulate the stem cell population and primary root growth in Arabidopsis.

Co-regulation of indole glucosinolates and camalexin biosynthesis by CPK5/CPK6 and MPK3/MPK6 signaling pathways^°°

Secondary plant metabolites,represented by indole glucosinolates(IGS)and camalexin,play important roles in Arabidopsis immunity.Previously,we demonstrated the importance of MPK3 and MPK6,two closely related MAPKs,in regulating Botrytis cinerea(Bc)-induced IGS and camalexin biosynthesis.Here we report that CPK5 and CPK6,two redundant calcium-dependent protein kinases(CPKs),are also involved in regulating the biosynthesis of these secondary metabolites.The loss-of-function of both CPK5 and CPK6 compromises plant resistance to Be.Expression profiling of CPK5-VK transgenic plants,in which a truncated constitutively active CPK5 is driven by a steroidinducible promoter,revealed that biosynthetic genes of both IGS and camalexin pathways are coordinately upregulated after the induction of CPK5-VK,leading to high-level accumulation of camalexin and 4-methoxyindole-3-ylmethylglucosinolate(4MI3G).Induction of camalexin and 4MI3G,as well as the genes in their biosynthesis pathways,is greatly compromised in cpks cpk6 mutant in response to Be.In a conditional cpk5 cpk6 mpk3 mpk6 quadruple mutant,Be resistance and induction of IGS and camalexin are further reduced in comparison to either cpks cpk6 or conditional mpk3 mpk6 double mutant,suggesting that both CPK5/CPK6 and MPK3/MPK6 signaling pathways contribute to promote the biosynthesis of 4MI3G and camalexin in defense against Be.

Sporophytic control of anther development and male fertility by glucose-6-phosphate/phosphate translocator 1(OsGPT1) in rice

Coordination between the sporophytic tissue and the gametic pollen within anthers is tightly controlled to achieve the optimal pollen fitness. Glucose-6-phosphate/phosphate translocator(GPT) transports glucose-6-phosphate, a key precursor of starch and/or fatty acid biosynthesis, into plastids. Here, we report the functional characterization of Os GPT1 in the rice anther development and pollen fertility. Pollen grains from homozygous osgpt1 mutant plants fail to accumulate starch granules, resulting in pollen sterility. Genetic analyses reveal a sporophytic effect for this mutation. Os GPT1 is highly expressed in the tapetal layer of rice anther. Degeneration of the tapetum, an important process to provide cellular contents to support pollen development, is impeded in osgpt1 plants. In addition, defective intine and exine are observed in the pollen from osgpt1 plants. Expression levels of multiple genes that are important to tapetum degeneration or pollen wall formation are significantly decreased in osgpt1 anthers. Previously, we reported that At GPT1 plays a gametic function in the accumulation of lipid bodies in Arabidopsis pollen. This report highlights a sporophytic role of Os GPT1 in the tapetum degeneration and pollen development. The divergent functions of Os GPT1 and At GPT1 in pollen development might be a result of their independent evolution after monocots and dicots diverged.

Induction ofγ-aminobutyric acid plays a positive role to Arabidopsis resistance against Pseudomonas syringae^°°

Gamma-aminobutyric acid(GABA)is an important metabolite which functions in plant growth,development,and stress responses.However,its role in plant defense and how it is regulated are largely unknown.Here,we report a detailed analysis of GABA induction during the resistance response to Pseudomonas syringae in Arabidopsis thaliana.While searching for the mechanism underlying the pathogen-responsive mitogenactivated protein kinase(MPK)3/MPK6 signaling cascade in plant immunity,we found that activation of MPK3/MPK6 greatly induced GABA biosynthesis,which is dependent on the glutamate decarboxylase genes GAD1 and GAD4.Inoculation with Pseudomonas syringae pv tomato DC3000(Pst)and Pst-avrRpt2 expressing the civrRpt2 effector gene induced GACH and GAD4 gene expression and increased the levels of GABA.Genetic evidence revealed that GAD1,GAD2,and GAD4 play important roles in bothGABA biosynthesis and plant resistance in response to Pst-avrRpt2 infection.The gad1/2/4 triple and gad1/2/4/5 quad-ruple mutants,in which the GABA levels were extremelylow,were more susceptible to both Pst and Pst-avrRpt2.Functional loss of MPK3/MPK6,or their upstream MKK4/MKK5,or their downstream substrate WRKY33 sup-pressed the induction of GAD1 and GAD4 expression afterPst-avrRpt2 treatment.Our findings shed light on both theregulation and role of GABA in the plant immunity to abacterial pathogen.